The main conditions essential for achieving very high efficiencies in photovoltaic solar cells are, in addition to optimum entrapment of the light by suitable surface structuring and contact arrangement, above all as low as possible a contact area and excellent surface passivation in the active area of the semiconductor. As a result, the short circuit current and--by lowering of the reverse saturation current--both the no-load voltage and the fill factor of the solar cell are increased. The highest laboratory efficiencies at present (around 24%) for silicon solar cells made with a doped pn-junction are achieved with a complex photolithographically generated surface substrate, and with high-temperature passivation by means of a thermal silicon dioxide layer in which very small openings are provided in turn by photolithography and etching for the metal contacts necessary to conduct the charge carriers (M. A. Green, S. R. Wenham, J. Zhao, J. Zolper and A. W. Blakers, Proceedings of 21st IEEE Photovoltaic Specialists Conference, p. 207, 1990). A double diffusion in the contact area contributes to a further reduction in the reverse saturation current. The complex manufacturing process is probably however only of limited viability for inexpensive mass production of terrestrial solar cells.
In the literature article "R. Hezel, W. Hoffmann and K. Jaeger, Proceedings of 10th E.C. Photovoltaic Solar Energy Conference, page 511, Lisbon 1991", a noteworthy MIS inversion layer solar cell is described that leads to a reduction in the manufacturing costs while achieving a high efficiency. Manufacture is achieved by simple low-temperature processes thanks to the induced pn-junction, and is hence suitable above all for very thin embodiments, including bifacial ones (sensitive to light on both sides). To increase the efficiency of the solar cell even further, a drastic improvement of the surface passivation is also necessary in addition to a reduction in the contact area.
It is known that the quality of the silicon surface passivation, and hence its efficiency too, can be increased by deposition of a plasma silicon nitride layer at around 450.degree. C. (W. Bauch and R. Hezel, Proceedings of 9th E.C. Photovoltaic Solar Energy Conference, p. 390, Freiburg, 1989). However, this requires two photolithographic steps in order to generate openings in the passivation layer and for definition of the contact grid, and these steps are only practicable with great difficulty for inexpensive mass production and for large solar cell areas.
From IEEE Electron Device Letters, Vol. 11, No. 1, January 1990, New York, p. 6-8, A. Cuevas et al., a point contact concentrator solar cell is made by photolithographic means. The solar cell surface has a large number of elevated areas of which only a few are covered in some areas by an electrically conducting material for forming a front contact. The electrically conducting material extends over areas of semiconductor material exposed in places and having a V-shaped cross-section, and in some areas over an oxide layer as a passivation layer. The electrically conducting material extends along both flanks of the elevated areas. Thanks to the photolithographic process steps, the manufacture of suitable cells is very complex and unsuitable for widespread commercial application on account of the high manufacturing costs, among other reasons.
A solar cell with strip-like front contacts is known from the 19th IEEE Photovoltaic Specialists Conference, May 4, 1987, New Orleans, La., U.S.A., p. 1424-1429, D. B. Bickler et al. The contacts extend exclusively over plateau-like areas of the semiconductor substrate that are covered in some areas by an oxide layer. To produce the structure, a large number of photolithographic process steps are necessary.
Both the solar cells described before and those described last require a complex masking, adjustment and etching technology.
Etching methods and masking technology are also necessary in a solar cell known from Applied Physics Letters, Vol. 55, No. 13, Sep. 25, 1989, New York, U.S.A., p. 1363-1365, A. W. Blakers et al., in which cell the front contacts are disposed in lines on an oxide layer having openings.
Structuring of a front surface of an MIS solar cell by anisotropic etching is also known (5th International Photovoltaic Science and Engineering Conference, Nov. 26, 1990, Kyoto, Japan, p. 701-704, R. Hezel et al.).
In the solar cell known from the 5th International Photovoltaic Science and Engineering Conference, Nov. 26, 1990, Kyoto, Japan, p. 533-536, H. Itoh et al., an a-Si solar cell substrate is applied to a prestructured glass carrier.
In Applied Physics Letters, Vol. 41, No. 7, Oct. 1, 1982, New York, U.S.A., p. 649-651, P. G. Borden et al., a Si solar cell is described in which a front contact structured by etching and photolithography covets a CVD oxide layer.
According to 14th IEEE Photovoltaic Specialists Conference, Jan. 7, 1980, San Diego, Calif., U.S.A., p. 783-785, T. G. Sparks et al., a surface structure of a solar cell is generated by anisotropic etching, and front contacts are deposited thereon by plasma etching and by mechanical masks.